GB2080781A

GB2080781A - Surface treatment for silicon carbide

Info

Publication number: GB2080781A
Application number: GB8122893A
Authority: GB
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1980-07-30
Filing date: 1981-07-24
Publication date: 1982-02-10
Also published as: DE3130117A1; FR2487732A1; JPS57111289A; FR2487732B1; US4340636A; DE3130117C2; JPH0260638B2; GB2080781B; CA1175308A

Description

1

SPECIFICATION

Surface treatment of silicon carbide GB 2 080 781 A 1 Background of the Invention

This invention relates to a coated stoichiometric silicon carbide and to a method of applying a carbon-rich surface layer to silicon carbide.

Unless otherwise qualified, silicon carbide shall mean stoichiometric silicon carbide.

Carbon-rich silicon carbide shall mean deposits in which the ratio of silicon to carbon shall be in the range of 0+ to l.

High-strength and/or high-modulus filaments or strips shall mean structures having a tensile strength of 300 ksi or greater and a tensile modulus of 40,000 ksi or greater.

The present invention is applicable to silicon carbide and carbon surfaces of all shapes and sizes. It is particularly important for filaments, then strips, and the like. The following discussion will be directed to filaments as a typical example.

Composite materials in plastics or metal matrices reinforced with highstrength, high-modulus filaments such as boron and silicon carbide are finding increased popularity in structural applications. In particular these types of composites are useful where high-strength and stiffness with accompanying low weight is desired.

The present state-of-the-art silicon carbide filament or thin strip contains a refractory core, generally 20 tungsten or carbon. The core may include an intermediate buffer zone followed by a relatively thick layer of silicon carbide. In general the silicon carbide and buffer zone are made by means of hydrogen reduction and chemical vapor deposition processes wherein gases containing silicon and carbon are decomposed and deposited on the core. The thickness of the silicon carbide coating is directly related to the deposition time and temperature.

An important use of said silicon carbide coatings is in connection with high-strength, high-modulus silicon carbide filaments of the type described in U.S. Patent No. 4,068,037. In this pateritthere is described a silicon carbide filament formed on a carbonaceous core. In other applications the silicon carbide coating is deposited on a tungsten core.

The aforementioned referenced patent, U.S. Patent No. 4,068,037, is the closest art known to Applicants in 30 relation to the present invention. The filament described in the referenced patent represents the state of the art. In particular, it will be noted that the filament described in the patent contains a carbon-rich silicon carbide outer coating which is important for maintaining overall filament strength and stiffness. However, this outer coating makes it very difficult to incorporate these filaments within metal matrices such as aluminum, titanium as well as epoxy matrices because the matrix material does not bond well to the 35 carbon-rich outer layer.

The outer carbon-rich silicon carbide layer of the state-of-the-art silicon carbide filament discussed above has been physically characterized as a layer where the ratio of silicon to carbon varies from one at the interface of the carbon-rich layer with the stoichiometer silicon carbide layer to zero at the exterior surface of the filament. In other words, the exterior surface of the carbon-rich layer is essentially pure carbon.

The industry has long known that it is extremely difficuitto incorporate carbon filaments within plastics and metal matrices. Carbon is, in one instance, highly reactive. Prior attempts to incorporate such state-of-the-art silicon carbide filaments containing carbon surfaces and/or carbon filaments within aluminum or titanium matrices by hot-molding have been less than desired. In most cases the resulting composite is not very strong because the moulding process has greatly weakened the filaments.

Additionally, carbon is not readily wetted by aluminum or titanium, or even by common plastics matrices such as epoxys. As a result composite properties suffer.

The difference in properties of the filament is composites between the heretofore state-of-the-art practice and the practice enumerated in this Application will become readily apparent with reference to the chart that is provided herein.

It is an object of the invention to provide a surface treatment for stoichiometric silicon carbide which enhances the wetting capability of the silicon carbide without deleterious effects on the strength of said silicon carbide.

It is yet another object of the invention to provide a method of making a high-strength, high modulus silicon carbide filament.

In accordance with the invention, there is provided a surface treatment for stoichiometric silicon carbide comprising a carbon-rich silicon carbide layer overcoating the stoichiometric silicon carbide. The ratio of silicon to carbon of the carbon-rich layer varies from one at the interface with stoichiometric silicon carbide to near zero at the interior to a value substantially greater than zero at the surface remote from said interface.

The carbon-rich silicon carbide treatment for carbon surface has a silicon to carbon ratio of zero at the carbon 60 surface to a value greater than zero in the remote surface.

It is hypothesized that silicon carbide is particularly sensitive to the presence of a non-stoichiometric silicon carbide or impurities. 1. T. Kendall, Journal of Chemical Physics, Vol. 21, pg. 821 (1953). Since both Kendall and K. Arnt & E. Hausmanne in ZeitsAnorg Chem., Vol. 215, pg 66 (1933) have found no evidence of non-stoichiometric silicon carbide, it is hypothesized that the excess carbon appears in the silicon carbide as 65 2 GB 2 080 781 A 2 an impurity. The properties of silicon carbide are particularly sensitive to the presence of impurities such as carbon.

Though the precise structure of carbon-rich silicon carbide may not be known with certainty, regions were quantitatively there is an excess of carbon have been observed.

The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in conjunction with the accompanying drawings, in which:

Brief Description of the Drawings

Figure 1 is a cross-sectional representation of silicon carbide filament embodying the principals of the present invention; Figure 2 is a curve which is useful in describing the invention; and Figure 3 is a schematic representation of a reactor for making silicon carbide filament.

is Description of the Invention

This invention will be described within the context of making silicon carbide filament. The invention is operative, irrespective of the structure and composition of the core used to make the filament. In addition, the invention has particular application to very thin strips of silicon carbide where it is desired to preserve the highstrength and high-modulus properties of thin strips.

Referring to Figure 1, there is shown a cross section of a silicon carbide filament embodying the principals of the present invention. More particularly, the filament contains a core 15 which may be tungsten, carbon, etc. In accordance with the teachings of U.S. Patent No. 4,068,037, the Figure 1 filament contains a carbon-rich buffer layer 30 on which a stoichiometric silicon carbide deposit 32 is provided. Wken the core 15 is a carbonaceous core, it is sometimes desirable to include a thin layer of pyrolitic graphite (not shown) 25 between the core 15 and the carbon-rich layer 30.

The silicon carbide filament in Figure 1 includes a surface deposit 34 of carbon-rich silicon carbide in accordance with the teachings of this invention. The cross section of the carbon-rich deposit 34 does not contain a uniform composition. The silicon to carbon ratio across the cross section of deposit 34 varies essentially radial ly in the case of this type of filament. The particular variation is shown in the Auger profile in 30 Fig u re 2.

Figure 2 is an Auger profile of the carbon-rich surface deposit 34. The composition of the surface deposit 34 is not uniform. Three regions 37,38 and 40 are apparent. At the surface 39 of region 37, the ratio of silicon to carbon is shown as 0.4 or 40%. The ratio of silicon to carbon in this region 37 drops rapidly away from the surface 39 and merges with a near pure carbon region 38. The ratio of silicon to carbon then rises almost 35 precipitously in region 40 until it achieves stoichiometric proportion at the interface between the layers 34 and deposit 32 at point A.

The silicon content is generally combined with carbon to form SiC in the carbon-rich surface layer. Some free silicon may exist howeverwithout detriment as aluminum and titanium matrices, in particular, wet silicon.

The overall depth of the carbon-rich surface layer 34 is generally from.7 to 1.3 microns. The depth of region 37 is generally 0.25 microns 20%, while the depth of region 38 is generally 0.5 microns. A minimum of 0.15 microns depth is recommended for region 40. Clearly, some variation in depth is permitted.

A workable ratio at the surface 39 is in the range of.3 to.5. This is not necessarily the optimum range.

However, experimental data indicates that between.3 and.5 a commercially viable filament results. In a 45 broad sense it is believed that any ratio more than zero will perform satisfactorily eitherfrom a strength point of view or a wetting point of view, or both.

The preferred method of making a silicon carbide is by means of a vapor deposition process. In Figure 3 of the drawings, there is shown schematically a reactor 10 which comprises a generally closed tubular cylinder 11 containing a pair of oppositely disposed closed ends 12 and 14. Central apertures containing mercury so contacts 16 and 18 are defined in each of the ends 12 and 14. The mercury contacts are coupled through terminals a-a to a source of electrical power not otherwise shown. The core 15 is obtained from a supply reel 20. The core 15 passes into a cylinder 11 into the mercury contact 16 and out of the cylinder 11 through the mercury contact 18 to a take-up reel 22. Briefly, the core 15 is raised to a deposition temperature by means of electrical resistance heating through terminals a-a in a conventional way.

A number of ports through which gas is fed to the cylinder 11 or exhausted from cylinder 11 are provided. The process for making a state- of-the-art silicon carbide filament shown in Figure 1 is fully descriffied in U.S. Patent No. 4,068,037, and such teaching is incorporated by reference herein. Typically, at the top of the reactor at port 24, a silane blend, hydrogen, argon, and propane are fed to the reactor in quantities to deposit on the core 15 the carbon-rich silicon carbide layer 30. Additional silane blend and hydrogen are added to dilute the mixture of gases in contact with the core 15 through the port 26. The mixture of gases is exhausted through port 28. Between ports 26 and 28 the silicon carbide deposit 32 is formed. The carbon-rich surface deposit 34 is produced by introducing argon, a silane gas, and propane through port 29. These gases are also exhausted through port 28. A baffle 31 may be provided to insure thatthe gases introducedIn port29 make contact with the filament; it is not a required structure. The preferred composition of the b.,L-nciewodng port v 9 3 GB 2 080 781 A 3 29 is 4 parts argon, 1 part propane and 0.02 parts dichlorosilane.

The silane gas is highly reactive relative to the propane so that it decomposes and deposits a carbon-rich silicon carbide deposit on the SiC 32 adjacent to the port 29. For purposes of illustration, we will assume that the silane gas decomposes in the region identified by the symbol x. Since the propane is less reactive than the silane, it will decompose further up the reactor, and for purposes of illustration, we will assume that the propane decomposes in the region y.

Thus as the filament moves from the supply reel 20 toward the take-up reel 22, it reaches the propane deposition region yfirst, and there is deposited on the surface of the filament at this point a carbon-rich deposit where the silicon to carbon ratio varies from one to essentially zero. As the filament enters the region x where the silanes are decomposing, the ratio of silicon carbide increases from essentially zero to a value 10 greater than zero as discussed above. The deposition conditions in this lower end of the reactor are similar to those set out in the patent referenced above.

Current experimental data indicates that a workable silicon to carbon ration at the exterior surface 39 is in the range of.3 to.5 with.4 exhibiting adequate results. It is clear that these are not necessarily the optimum results. There is no reason to believe that the silicon to carbon ratio at the exterior surface 39 cannot increase 15 or decrease from the presently known workable range.

Representative properties of the heretofore state-of-the-art filament and composites in comparison with similar properties of the new filament are provided in the chart below.

Properties 20 PriorArt New 25 Filament Tensile Strength 560-600 ksi (scatter) 720 ksi average Surface Strength (Loop Dia) 8-10 mm 8-10 mm Castable No - 60-80 ksi Greater than 200 ksi 30 Diffusion-bonding - 5000 psi <200 ksi; 30 min. cycle Greater than 200 ksi; min. cycle Hot-molding - 400-800 psi No consolidation Greater than 200 ksi 35 Dipped in molten aluminum 1250F Degradation after 15 min. 3 hrs - no degradation Dipped in molten aluminum 140OF Degradation after 5 min. 30-60 min. - no degratation 40 The various features and advantages of the invention are thoughtto be clearfrom the foregoing description. Various otherfeatures and advantages not specifically enumerated will undoubtedly occurto those versed in the art, as likewise will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and and scope of the invention as defined by the following claims:

Claims

1. A coated stoichiometric silicon carbide comprising:

a first surface comprising stoichiometric silicon carbide, and a layer of carbon-rich silicon carbide 50 contiguously overlying said first surface, wherein the ratio of silicon to carbon of said layer varies from one at the interface with said first surface to near zero at the interior of the layer to greater than zero at the surface of said layer remote from said interface.

2. A coated stoichiometric silicon carbide as claimed in Claim 1, wherein the ratio of silicon to carbon at said remote surface is from.3 to.5.

3. A coated stoichiometric silicon carbide as claimed in Claim 2, wherein said ratio is 0.4 at said remote surface.

4. A coated stoichiometric silicon carbide as claimed in Claim 1 or 2, wherein the thickness of said carbon-rich layer is from 0.7 to 1.3 microns.

5. A silicon carbide filament or strip comprising:

a core; an inner surface layer of carbon-rich silicon carbide surrounding said core; a coating of silicon carbide on said inner surface layer of carbon-rich silicon carbide layer; and an exterior carbon-rich silicon carbide layer overlying the silicon carbide in a contiguous fashion, wherein the ratio of silicon to carbon of said exterior surface varies from one to the interface with said first surface to 65 4 GB 2 080 781 A 4 nearzero atthe surface of said layer remotefrom said interface.

6. A filament or strip as claimed in Claim 5, wherein the inner surface layer of carbon-rich silicon carbide has a thickness of from 0.7 to 1.3 microns.

7. A filament or strip as claimed in Claim 5 or 6, wherein the core is carbonaceous.

8. A filament or strip as claimed in Claim 7, wherein a layer of pyrolytic graphite is provided between the 5 core and the carbon rich inner surface layer.

9. A filament or strip as claimed in Claim 5 or 6, wherein the core consists essentially of tungsten.

10. A filament or strip as claimed in anyone of Claims 5 to 9, wherein. the region wherein the ratio of silicon to carbon changes from one to near zero has a depth of at least 0. 15 microns.

0

11. A filament or strip as claimed in anyone of Claims 5 to 10, wherein the region wherein the ratio of 10 silicon to carbon is near zero has a depth of approximately 0.5 microns.

12. A filament or strip as claimed in anyone of Claims 5 to 11, wherein the region wherein the ratio of silicon to carbon changes from near zero to greater than zero has a depth of 0.25 microns 20%.

13. A filament or strip as claimed in anyone of Claims 5 to 12, wherein the ratio of silicon to carbon at the surface of said layer remote from said interface is from 0.3 to 0.5.

14. A filament or strip as claimed in Claim 13 wherein said ratio is approximately 0.4.

15. A silicon carbide filament or strip, substantially as hereinbefore described, with reference to Figures 1 and 2 of the accompanying drawings.

16. A method of applying a carbon-rich surface layer to silicon carbide, which method comprises:

supplying a blend of a hydrocarbon and a silane to a reactor containing a heated silicon carbide surface; 20 subjecting the silicon carbide surface predominantly to the hydrocarbon to vapour-deposit a carbon-rich region wherein the ratio of Si/C is 1 at the interface of the SiC surface and drops to near zero adjacent to said interface; and subsequently increasing the proportion of silane relative to hydrocarbon to vapour-deposit increasing amounts of SiC so asto increase the Si/C ratio whereby there are created in the carbon-rich layer three portions, one in which the Si/C ratio is decreasing, a second where the ratio is near zero and a third where the ratio is near zero and a third where the ratio is increasing to a second maximum at the exterior surface.

17. A method as claimed in Claim 16, wherein the Si/C ratio at the surface of the carbon-rich layer is from 0.3 to 0.5

18. A method as claimed in Claim 17, wherein the Si/C ratio at the surface of the carbon-rich layer is 0.4.

19. A method as claimed in anyone of Claims 16 to 18, wherein the depth of the carbon-rich layer is from 0.7 to 1.3 microns.

20. A method as claimed in Claim 19 wherein the depth of the third portion is 0.25 microns 20%.

21. A method as claimed in anyone of Claims 16 to 20, wherein the hydrocarbon consists essentially of 36 propane and the silane consists essentially of dichlorosilane.

22. A method of applying a carbon-rich surface layer to silicon carbide filaments which method comprises:

passing a silicon carbide filament though an elongate reactor having an upstream entrance port; supplying a blend of a hydrocarbon and a silane to said downstream entrance port containing a heated 40 silicon carbide filament surface; subjecting the silicon carbide surface adjacent said exit port predominantly to the hydrocarbons to vapour deposit a carbon-rich region wherein the ratio of Si/C is 1 at the interface of the SiC surface and drops to near zero adjaceritto said interface; and increasing the proportion of silane relative to hydrocarbon adjacent to said entrance port to vapour-depositing increasing amounts of SiC so to increase the Si/C ratio, whereby there are created in the carbon-rich layer three portions, one in which the Si/C ration is decreasing, a second where the ratio is near zero and a third where the ratio is increasing at the exterior surface.

23. A method as claimed in Claim 22, wherein the hydrocarbon is propane and the silane is dichlorosilane.

24. A method as claimed in Claim 23, wherein the blend of hydrocarbon and si lane is 4 parts argon, 2 part propane and 0.02 parts dichlorosilane.

25. A method of applying a carbon-rich surface layer to silicon carbide filaments or strips, substantially as hereinbefore described with reference to the accompanying drawings.

26. The features hereinbefore disclosed, or their equivalent, in any novel selection.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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